tokamak

An experimental arrangement for achieving controlled nuclear
fusion. In a tokamak, two superimposed magnetic
fields enclose the plasma: this is the
toroidal field generated by external coils on the one hand and the field
of a flow in the plasma on the other hand. In the combined field, the field
lines run helicoidally around the torus centre. In this way, the necessary
twisting of the field lines and the structure of the magnetic areas are
achieved.

Apart from the toroidal field generated by the external field coils and
the field generated by the flow in the plasma, the tokamak requires a third
vertical field (poloidal field), fixing the position of the flow in the
plasma container. The flow in the plasma is mainly used to generate the
enclosing magnetic field. In addition, it provides effective initial heating
of the plasma. The flow in the plasma is normally induced by a transformer
coil.

Owing to the transformer, the tokamak does not work continuously, but in
pulse mode. Since, however, a power plant should not be operated in pulse
mode for technical reasons, methods are examined to generate a continuous
flow – for example by high-frequency waves.

The fusion research plant JET is built according to the tokamak principle.
The fusion reactor ITER is also planned according to this principle.